I have not had any experience with Sibelius at all, but the first thing I would look for is the relative volume of the two patches in the Kontakt player. Perhaps adjusting one of these would help compensate for the difference. I haven’t found this to be a problem with DP on a Mac, but as I mentioned I’ve had no experience with Sibelius whatsoever.
I could be mistaken, but I think the pizz. articulations don't respond to CC#1, they respond to velocity.
Exactly right. There are two broad categories in GPO: Expressive and Percussive. Expressive involves sustained a notes and control is provided through mod wheel data. Percussive involves "strike and decay." Piano is a good example of a Percussive instrument. Pizzicato strings are another example. Percussive instruments use standard velocity response to control volume/brightness. Most of the time in GPO the categories are independent of one another but they can occur within the same instrument when keyswitch selections are available. When you choose the pizz KS, use higher velocities if you want the notes to be louder.